The quantitative answer is: about 7% more IBUs at 90 minutes vs. 60 minutes. Or conversely, a 60-minute boil will give you about 93% as much IBUs as a 90-minute boil.
Source: Hop utilization table from How To Brew by Palmer, shown in the green & white table here:
https://www.homebrewtalk.com/threads/formulating-balanced-beer-recipes.679116/Assumption: OG 1.060 (pretty close, right?)
Simply divide the utilization factor of 0.226 @90 minutes by the factor of 0.211 @60 minutes to understand the effect.
If I use an IBU formula at each timing for the Galena hops you specified, 0.5 oz 13% aa, with an assumption of a 5-gallon batch, I get 25.7 IBUs at 90 minutes and 23.6 IBUs at 60 minutes. Then 23.6/25.7 = 92%. Or the inverse would be an extra 9% IBUs at 90 minutes vs. 60 minutes. Oh look, my numbers work out pretty close.
But of course, people will say "this is only calculated, you would have to measure in a lab to know for sure" or "you probably can't taste the difference anyway". But in my experience.... I would say, this is a pretty damn close estimate of reality.
And bottom line: Yeah, while you might get a couple more IBUs, you probably can't taste the difference.
Also, Deschutes might not boil 90 minutes. That is indeed expensive. A clone recipe might choose to do so, because it wasn't written by Deschutes but a homebrewer probably, doesn't cost a homebrewer much to boil a little longer if they want an extra couple IBUs just to lean on the safe side or for whatever other reasons (e.g., DMS concerns which are largely baloney).
To rephrase and reemphasize the bottom line: Feel free to boil for 60 minutes instead of 90. It's not gonna matter.